1. Field of the Invention
The invention pertains to fiber optic devices and more particularly to fiber optic structural modal sensors for monitoring the dynamic properties, such as vibration characteristics, of structures.
2. Description of the Prior Art
The advent of the shuttle transport system initiated consideration of a unique class of earth orbiting structures for on-orbit assembly. These structures may be very large and flexible, for on-orbit assembly does not require characteristics to withstand launch vibrations or lg environments. To yield stable space platforms the flexure of these structures in the low frequency, high amplitude structural bending modes must be controlled. This control may be accomplished with various discreet actuators mounted on the structure. Identification of these structural modes and determination of modal excitation present difficult problems. Discreet sensors such as accelerometers, inertial reference platforms, and laser interferometers can detect structural mode excitation. The discreet nature of these devices, however, provide only localized information for the actuator system, thus many sensors per structure are required to maximize modal sensing capability.
Vibration monitoring devices may be desirable in systems other than space platforms. Helicopters, for example, possess large rotor blades that rotate at relatively high angular velocities. These rotation rates cause the blades to vibrate and flex, which under adverse conditions, may cause the blades to fail, thereby creating a hazardous situation.
Optical systems for the detections of variation in ambient conditions that utilize optical fibers excited by coherent light have been considered for structural vibration mode detecting sensors. An acoustic sensor of this type for sonar detection has been proposed by M. R. Layton and J. A. Bucaro in an article entitled, "Optical Fiber Acoustic Sensor Utilizing Mode/Mode Interference", Applied Optics, Vol. 18, No. 5, Mar. 1, 1979, and a strain gage utilizing this concept has been disclosed in U.S. Pat. No. 4,191,470 issued to C. T. Butler in March 1980.
Butler discloses a fiber optic strain gage wherein parallelly spaced, single mode optical fibers are attached to a beam for the length thereof to measure the strain thereon. Coherent light coupled to the fibers propagates therethrough to a detector responsive to light emitted therefrom. Strain or motion of the beam is determined by detecting the motion of an interfering pattern produced by phase difference variations of the light transmission in the fibers. This arrangement is relatively complex mechanically and optically; requiring two fibers that must be attached to the structure under observation and precision coupling of optical energy to and from the single mode fibers.
The Layton et al. acoustic detector utilizes a statistically positioned long length of multi-mode optical fiber wound into a coil. Coherent light, coupled to the input end of the optical transmission line in an optical detector, is coupled to the output end. Compression of the fiber diameter, caused by the acoustic waves, establish phase variations between propagating modes, thereby altering the light patterns produced by the interference between at least two of the modes. The optical fiber coil of Layton et al. is static, mode transmission characteristics being varied by the compression of the fiber diameter. Positioning such a device to detect structural vibrations presents a nearly insoluble problem.